Printers generally apply marking substances (e.g., inks) to receivers (e.g., paper). Inks used in inkjet printers are generally hydrophilic, and include a solute (e.g., pigment particles or dye molecules) dissolved or suspended in an ink solvent (e.g., water). The solvent in an ink needs to be removed to form a permanent image. Moreover, the solvent can soak into a receiver, causing the receiver to lose strength or mechanically deform. The solvent's soaking into a receiver, especially a fibrous receiver such as paper, can also reduce image quality by reducing effective resolution (because the ink spreads) and reducing density (the color of the fibers can show through as the ink soaks in to the receiver). It is therefore desirable to dry the ink rapidly to reduce absorption of the ink into the marked receiver. Drying can remove solvent dissolved into the receiver, or remove solvent from ink drops that have not yet permeated or dissolved into the receiver. In many printers, drying is the step that determines the speed at which a printer can operate. It is therefore desirable to dry as quickly as possible to increase printer productivity.
Various schemes have been described for drying inks on a marked receiver. Many dryers blow hot air across a wet image on a receiver. However, air has a low heat capacity, which limits its ability to transfer heat. Moreover, the hot air transfers heat not just to the ink, where the heat is desired, but also to the receiver. This failure to concentrate the applied heat can slow down the drying process. It is also desirable to keep the temperature of paper receivers low, limiting the thermal power that can be applied. Moreover, blowing hot air can smear the ink that is either being jetted or is on the receiver, thereby degrading the image.
Other schemes include irradiating the marked receiver (e.g., with infrared or microwave radiation). However, in order to avoid excessive heat absorption in the receiver, the frequency must be carefully chosen. Moreover, many receivers contain some water under normal conditions, as atmospheric moisture falls down its concentration gradient into dry porous or semi-porous sheets. Accordingly, it may not be possible to heat the ink without also heating the receiver.
Furthermore, drying different areas of a receiver at different rates can result in wrinkling or distortion of the receiver. These problems can worsen as the speed of drying increases, or when the receiver is locked in place (e.g., in a nip) while drying. Various schemes require drying parameters be adjusted according to the type of media used (e.g., coated vs. uncoated paper). Moreover, the moisture released during drying can condense on surfaces in a printer. Drying can also cause paper, especially semi-porous paper, to blister: water within the paper can vaporize, creating sufficient pressure to disrupt the surface of the paper.
Various ways of removing substances from receivers have been described. U.S. Pat. No. 4,654,980 to Bhat, entitled “Solvent removal using a condensable heat transfer vapor,” describes removing non-aqueous solvents from a receiver by applying a countercurrent of saturated steam. U.S. Pat. No. 5,172,709 to Eckhardt et al., entitled “Apparatus and process for removing contaminants from soil,” describes removing contaminants (e.g., oils or heavy metals) from a substrate material (e.g., soil) using a hot pressurized liquid (e.g., steam). However, these schemes use water to remove non-water. Inkjet drying involves removing water or another aqueous solvent while retaining the non-water. These schemes are therefore unsuitable for inkjet drying.
Various schemes have also been described to improve the application of material to receivers. Some schemes using purpose-made coated inkjet papers to improve drying performance. However, these schemes inherently limit the types of paper that can be used, and coated inkjet papers are generally more expensive than standard commercial papers. U.S. Pat. No. 6,309,463 to Hess et al., entitled “Device for direct or indirect application of liquid or viscous coating medium onto a moving material web,” deliberately moistens a material to permit a coating to smooth and bond more effectively to the material. This can include directing hot liquid vapor towards the paper. However, drying involves removing moisture, not adding it. Causing coating material to adhere more effectively to a substrate does not assist with removal of moisture from that substrate.
U.S. Pat. No. 4,943,816 to Sporer, entitled “High quality thermal jet printer configuration suitable for producing color images,” discloses the use of a marking fluid containing no dye so that a latent image in the form of fluid drops is formed on a piece of paper. The marking fluid is relatively non-wetting to the paper. Sporer teaches the use of a 300 dpi thermal inkjet printer to produce the latent image. Surface tension then causes colored powder to adhere to the fluid drops. Sporer teaches that only that portion of the droplet that has not penetrated or feathered into the paper is available for attracting dry ink, so this process is unsuitable for highly-absorbent papers such as newsprint. Moreover, this process does not remove moisture from the receiver, so drying can still be required. Also, this process is a hybrid of inkjet and powder printing, so is not suitable for use in conventional inkjet printers.
There is, therefore, a continuing need for ways of removing moisture from receivers, e.g., to permit producing high-quality images at high speed using inkjet printers.